This invention relates generally to color analysis in manufacturing, and in particular, to the determination of the degree of oxidation of a copper oxide coating by color analysis.
Dumet wire is used in sealing lead-in wires for lamps or electronic devices. The thickness of the copper oxide coating of Dumet wire varies according to the degree of the oxidation, and the degree of oxidation is closely related to the sealability of Dumet wire to glass. To generate various grades of Dumet wire for different types of secondary processing, like welding or sealing with other materials, it is necessary during the manufacturing process to evaluate and then control the degree of oxidation of the Dumet wire based on the thickness of its oxide coating.
The thickness of the oxide coating is on the order of several microns and such fine thicknesses make it extremely difficult to test each product. It is therefore customary to classify the degree of oxidation on the basis of the color of the oxide coating since that color varies from reddish yellow to dark red depending upon the coating's thickness. The variation of color with oxidation is extremely subtle and the hue, saturation, and lightness of the color do not always vary regularly. Classification techniques which involve the visual comparison of the oxide color with a series of color samples require much time and labor and are not very accurate. Thus, more automatic techniques have been used to examine the color of the oxide.
Oxidation colors may be expressed numerically in such psychophysical quantities of color as tristimulus values X, Y, and Z, as well as by chromaticity coordinates x and y. The tristimulus values have been standardized by the Commission Internationale de l'Eclairage (CIE) and are defined as follows for a given sample having a spectral reflectance .rho.(.lambda.) and which is illuminated by a light source having a spectral power distribution S(.lambda.): ##EQU1## where K is a normalizing factor equal to ##EQU2##
In the equations for the tristimulus values shown above, Y is specifically referred to as luminous reflectance. Also, expressions x(.lambda.), y(.lambda.), and z(.lambda.) are referred to as CIE color matching functions and the CIE has assigned the numerical values to varying wavelengths shown in FIG. 1.
The chromaticity coordinates x, y are defined as follows: ##EQU3##
By determining the spectral reflectance .rho.(.lambda.) of a given sample with a spectrophotometer, the tristimulus values X, Y, and Z, the chromaticity coordinates x and y, and the luminous reflectance Y can be calculated on the basis of the known data of the spectral distribution S(.lambda.) of the light source used for illumination. This method of determination is known as spectrophotometric colorimetry.
Another method for determining the values mentioned above is called photoelectric tristimulus colorimetry and uses three photodetectors each of which includes color filters combined with a photocell. These filters possess spectral sensitivities matched to the values of the color matching functions x(.lambda.), y(.lambda.), and z(.lambda.). In this method, the tristimulus values are determined directly from the outputs of the three photodetectors. Photoelectric tristimulus colorimetry, however, suffers from poor reproducibility in the photodetectors since the color filters used to match the sensitivities of the photodetectors to the color matching functions generally possess low transmitting levels. Also, it is extremely difficult to match color filters to the color matching functions exactly. Because of these disadvantages, most investigators who have needed highly accurate measurements have had to use the spectral photometric method described above despite its complexity, expense and difficulty.
In the spectrophotometric method, colorimetric values such as the chromaticity coordinates x and y and the luminous reflectance Y of the copper oxide coating should correspond to the degree of oxidation of copper. FIG. 2 shows the distribution of chromaticity, specifically coordinates x and y, values for nine copper oxide coating samples, a-i, each having a different degree of oxidation. Sample a has the smallest degree of oxidation and sample i has the greatest. As FIG. 2 demonstrates, although samples a-i show mutually approximating trends, they exhibit no conspicuous regular variation, except the slight indication that the chromaticity coordinate y decreases slightly with increasing oxidation degree. Thus, even using the colorimetric instrument in the spectral photometric method, it is extremely difficult to determine the degree of oxidation of a copper oxide coating accurately using chromaticity coordinates x and y.
Therefore, an object of this invention is apparatus and methods to determine the degree of oxidation of copper oxide coating easily and accurately.
Additional objects and advantages of this invention are set forth in part in the following description and in part are obvious from that description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by the methods and apparatus in the appended claims.